In the Anthropocene, one of the clearest signatures of biodiversity change is rapid reorganization in species composition through time, hereafter termed temporal turnover. Climate change is often inferred to accelerate turnover in marine communities. Faster mean annual warming is frequently associated with faster turnover. Yet this pattern does not explain why turnover varies so widely among regions experiencing similar mean annual warming. A plausible explanation is seasonality. Communities may be constrained by their upper and lower thermal limits. Summer and winter extremes can impose different selective and demographic pressures. If so, the same annual mean warming can arise from very different seasonal trajectories, with different consequences for turnover. To test this idea, we collected global estimates of temporal turnover across marine assemblages. We then quantified season-specific warming. Stronger summer warming was associated with slower turnover. In contrast, stronger winter warming was associated with faster turnover. These opposing effects point to asymmetric warming as a key source of global heterogeneity in turnover. This asymmetry may act through differences in poleward range shifts, a dominant mechanism of community reassembly. Taken together, our results show that seasonal warming patterns refine expectations based on annual means. They also suggest that greenhouse-gas–driven amplification of summer warming may broadly suppress community turnover, increasing vulnerability under continued climate change.